The invention relates to a two-stroke engine and especially a two-stroke engine in the form of a drive motor in a portable handheld work apparatus such as a motor-driven chain saw, brushcutter, cutoff machine or the like.
Two-stroke engines of the above kind are generally known. To reduce the system-caused scavenging losses when exchanging charge, it has already been suggested to advance fuel-deficient or fuel-free air in advance of the inflow of the fresh mixture into the combustion chamber so that the scavenging losses are essentially formed by the fuel-free gas. The scavenging losses are unavoidable because of the open outlet which is slot controlled.
To introduce the advance air it is known to provide that the air channel opens into the transfer channel close to the transfer windows so that, during the induction phase, because of the underpressure in the crankcase, not only the mixture is drawn in via the inlet but, at the same time, fuel-free air flows in via the air channel from the transfer window in the direction toward the crankcase. At the start of a scavenging cycle, the fresh mixture, which flows in from the crankcase, must first displace the air from the transfer channel into the combustion chamber. The advance air, which is drawn in during an induction phase, is displaced into the combustion chamber for scavenging directly following the induction phase.
The constructive complexity for supplying the transfer channels with pure air is considerable. It must be ensured that the residual constituents of the fresh mixture, which are still present in the transfer channel, are scavenged completely by the in-flowing fuel-free air to the crankcase during the induction phase. If parts of the fresh mixture remain in the transfer channel, the scavenging losses are richer in energy and the quality of the exhaust gas deteriorates.
It is an object of the invention to provide a two-stroke engine which is improved in such a manner that a complete scavenging of the transfer channel with fuel-free air is ensured with less complexity.
The two-stroke engine of the invention includes an engine in a portable handheld work apparatus. The two-stroke engine of the invention includes: a cylinder having a cylinder wall; a piston mounted in the cylinder to undergo a reciprocating movement along a stroke path between top dead center and bottom dead center during operation of the engine; the cylinder and the piston conjointly delimiting a combustion chamber; a crankcase connected to the cylinder; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder; an outlet for conducting exhaust gases away from the combustion chamber; a transfer channel connecting the crankcase to the combustion chamber; the transfer channel having a first end defining a transfer window lying in the cylinder wall and opening into the combustion chamber; the transfer channel having a second end defining an inflow opening; a mixture-preparation unit for supplying an air/fuel mixture; an inlet in the crankcase downstream of the mixture-preparation unit for conducting the air/fuel mixture into the crankcase; an air channel for supplying an essentially fuel-free gas flow to the crankcase; a storage space in the crankcase; the air channel communicating with the crankcase through the storage space; and, a valve mounted in the crankcase so as to be movable between a first position wherein the inflow opening of the transfer channel is connected directly to said crankcase and a second position wherein the inflow opening is connected indirectly to the crankcase via the storage space.
The air channel supplying essentially fuel-free gas, such as air, opens into the crankcase via a storage space. The air channel advantageously opens in the foot region of the transfer channel at the elevation of the inflow opening into the crankcase. The inflow opening can be flow connected via a valve either directly to the crankcase or indirectly to the crankcase via the storage space.
A control is provided by means of the valve whereby, at the end of a scavenging cycle, the direct connection of the inflow opening of the transfer channel to the crankcase is blocked and an indirect connection is established to the crankcase via the storage space. In the induction phase, the storage space is filled with fuel-free air. For this reason, the fresh mixture, which enters the storage space because of the pressure conditions in the crankcase, displaces the air via the valve into the transfer channel. The transfer channel is completely filled with air from the storage space starting at the inflow opening and extending to the transfer window. All residual constituents of fresh gases, which remain in the transfer channel, are displaced into the combustion chamber. No reversal of the flow direction in the transfer channel is necessary for this scavenging thereof. Before the air, which is displaced into the transfer channel from the storage space, can, in turn, enter into the combustion chamber via the transfer window, the piston has closed the transfer window so that the fuel-free air remains as advance air in the transfer channel. The valve maintains the connection of the inflow opening to the air channel and to the storage space open even in the next induction phase, that is, the transfer channel or the inflow opening thereof continues to be only indirectly connected to the crankcase.
During the induction phase, the storage space again fills with fresh air. The fresh mixture, which has entered into the storage space, is scavenged into the crankcase. When the transfer window opens for the next scavenging cycle, the valve again switches into the second position wherein the inflow opening is connected directly to the crankcase. The fresh is mixture, which passes from the crankcase into the transfer channel, displaces the air, which is stored in the transfer channel from the previous scavenging cycle, into the combustion chamber. It is ensured that the entering advance air is free of fuel-rich residual gases.
A complete scavenging of the transfer channel with fresh air can be guaranteed by the selected constructive configuration and with low complexity without flow reversal. This is essentially achieved in that the advance air is displaced into the transfer channel at the end of a scavenging cycle whereby all residual gases of the previous scavenging cycle are scavenged into the combustion chamber. The advance air of the scavenging cycle remains in the transfer channel and is available for the next scavenging cycle, that is, one work stroke later.
It is practical to provide that the valve is a valve which is controlled in dependence upon the rotational position of the crankshaft. The valve is especially a mechanical multipath valve positively-controlled by the crankshaft. For this purpose, it is advantageous to configure the crankweb as a valve member in the form of a rotating disc.
In a preferred embodiment, the transfer window to the crankcase is opened shortly ahead of the top dead center in order to achieve a longer scavenging of the transfer channel and at about the same time, the storage space is blocked from the crankcase. In this way, fresh air can flow via the valve into the transfer channel toward the crankcase and scavenge residual gases.